Solar
Collector area, derived instead of guessed
Every solar pool heating page you will find says to install collector area equal to somewhere between half and all of your pool's surface area. That range is so wide it is not advice. The area you need depends on how much sun your site gets in the month you care about, how far above the air you are trying to hold the water, and what curve your particular mat sits on — and all three of those are computable.
An unglazed collector is a very steep curve
A solar collector's useful output follows its efficiency curve:
η = FR(τα) − FRUL × (Tin − Tair) / G
The first term is what fraction of the light it captures when the fluid is at air temperature. The second is what it loses to the surroundings, scaled by how far above air temperature you are running and inversely by how bright the sun is.
Unglazed polymer mats — the black tubing panels used for pools — have a high first term and a brutal second one, because there is no glazing between the absorber and the wind. Values around 0.75 and 20 W/m²K are representative of the class. That combination is excellent when you are only lifting the water a few degrees above air, which is exactly the pool case, and useless the moment the air gets cold.
How much collector your pool needs
Certified collector ratings publish both curve numbers for a specific product. If you have them, enter them and this stops being a class estimate.
Collector efficiency in that month 83%at 1,248 W/m² mean irradiance
Useful output per square foot 1,971 Btu/dayfrom the collector curve, not a rule of thumb
Collector area required 108 sq ftto carry that month unaided
As a share of pool surface 21%the number the rules of thumb are guessing at
The same array across your season
| Month | Mean air | Sun (kWh/m²/day) | Collector efficiency | Area to carry the month | Share of pool area |
|---|---|---|---|---|---|
| April | 69.1 °F | 7.28 | 61% | 1,166 sq ft | 228% |
| May | 78.7 °F | 8.17 | 71% | 655 sq ft | 128% |
| June | 89.5 °F | 8.50 | 79% | 356 sq ft | 70% |
| July | 93.0 °F | 7.49 | 83% | 108 sq ft | 21% |
| August | 91.3 °F | 6.91 | 82% | 123 sq ft | 24% |
| September | 85.9 °F | 6.16 | 77% | 524 sq ft | 102% |
| October | 74.2 °F | 5.01 | 62% | 1,630 sq ft | 318% |
The shape of that last column is the honest answer to "how much solar do I need". In the middle of the season the requirement is modest. At the shoulders it climbs, sometimes past the point where any array that fits on the roof can do the job, because the collector efficiency and the available sunlight are both falling while the pool's demand is rising.
This is why solar pool heating has a reputation for being either brilliant or disappointing depending on who you ask. Both groups are right about their own months.
Things the area calculation does not include
- Roof orientation and tilt. The irradiance figure used here is on a horizontal surface. A south-facing array at a shallow tilt beats it; a north-facing roof in the northern states does considerably worse. Correcting for that properly needs hourly solar geometry, which a monthly climate mean cannot support, so it is left out rather than approximated badly.
- Shading. One tree can remove a third of the resource from a specific array and nothing in a climate dataset knows about it.
- Pump energy. Pushing water up to a roof array and back costs pumping power that a gas heater does not need. On a variable-speed pump the increment is small; on a single-speed pump forced to run at high head, it is not.
- Freeze protection. Anywhere that freezes, the array needs draining and the controls need to handle it. That is an install requirement, not an optional extra.
What solar is genuinely good at
Solar is the only heat source on this site with no fuel cost at all, and unglazed mats have very few things in them that fail. For a pool in a sunny climate where the owner wants a warmer mid-season rather than a longer season, it is hard to beat: the collector runs at its best exactly when the demand is easiest, and the marginal cost of every degree is zero.
Where it disappoints is the request people usually make of it — extending the season into April and October. In those months the curve above is at its worst and the demand is at its highest. Pairing a modest array with a heat pump for the shoulders is a more honest configuration than an enormous array trying to do the shoulders alone.